The incubation period for rabies is typically 2-3 months however; it can range from 7 days to 1 year. The incubation period depends on factors inclusive of: location and severity of bite wound, the amount of virus inoculated within the wound and degree of innervation at the site of the bite wound.
No clinical signs are characteristic of rabies and differential diagnosis may involve many agents or syndromes. Initial symptoms often include fever, pain, unusual or unexplained tingling, pricking or a burning sensation (paraesthesia) at the wound site.
As the virus spreads from muscle cells at the site of inoculation through the peripheral nerves to the central nervous system, progressive, fatal inflammation of the spinal cord and subsequently, brain (encephalitis) develops.
Rabies infection manifests in two forms of rabies: furious (classical or encephalitic) form and the paralytic form.
- Furious rabies accounts for approximately 80% of the total number of human cases; manifesting as signs of hyperactivity, hypersalivation, periods of agitation alternating with lucidity, hydrophobia and sometimes aerophobia. After a few days, the infection invariably leads to coma and death by cardio-respiratory arrest.
- Paralytic (or dumb) rabies accounts for approximately 20% of the total number of human cases. This form of rabies runs a less dramatic but longer course than the furious form. There is flaccid muscle weakness in the early onset of infection, starting at the site of the bite or scratch, followed by gradual paralysis. Death by respiratory failure is generally preceded by the development of a coma. The paralytic form of rabies is often misdiagnosed, contributing to the underreporting of the disease.